Digital techniques for audio signal processing extend to the driving of audio output amplifiers. A popular class of amplifier circuits in many audio applications are “class D” amplifiers. Class D amplifiers drive a complementary output signal that is digital in nature, with the output voltage swinging fully from “rail-to-rail” at a duty cycle that varies with the audio information. Complementary metal-oxide-semiconductor (CMOS) output drive transistors are thus suitable for class D amplifiers, as such devices are capable of high, full-rail, switching rates such as desired for digital applications. CMOS drivers conduct extremely low DC current, and their resulting efficiency is especially beneficial in portable and automotive audio applications. In addition, the ability to realize the audio output amplifier in CMOS enables integration of an audio output amplifier with other circuitry in the audio system, further improving efficiency and also reducing manufacturing cost of the system. This integration also provides performance benefits resulting from close device matching between the output devices and the upstream circuits, and from reduced signal attenuation.
As is known in the art, an undesirable effect of a direct current (DC) component can exist in the output of a class D amplifier. The DC component can cause audible artifacts, such as a “click/pop” noise, an audible “humming” noise or a crunching sound. Detection of such an undesirable DC component is thusly needed.